1. Field of the Invention
This invention relates to a cooking appliance having the dual function of hot air circulation heating and microwave energy dielectric heating. More particularly, this invention relates to a cooking appliance comprising a heating chamber and a compartment adjacent to said heating chamber, said compartment accommodating a heater means for elevating the atmospheric temperature within said heating chamber and a fan means for circulating the air through said heating chamber and compartment.
2. Description of the Prior Art
The cooking appliances of the hot air circulating type which have so far been available on the market are not equipped with means for varying the rotational speed of a air-circulating fan but rather the fan speed is substantially constant. Therefore, in the appliances designed mainly for making confectionary items, the usual arrangement calls for the lowest possible fan speed in order to reduce the velocity of hot air flow so as not to cause the food to dry. As a consequence, when a foodstuff such as a whole poultry or its thigh or the like is to be cooked, the low air velocity means prolongs cooking and it takes a fairly long time to brown the surface of the food. And a substantial portion of the broth seeps out thereby impairing the flavor of food.
On the other hand, a cooking appliance designed mainly for the cooking of meat has the disadvantage that when it is used for making confectonary items, the air velocity is so high that the surface of food will dry.
Furthermore, in the prior art cooking appliances, the path of circulating hot air comprises several comparatively large openings bored through the wall of a heating chamber, and when a microwave heating means is additionally incorporated in such an appliance, choke structures must be provided near the drive shaft of the circulating fan, the heater connection, etc., which entails an additional cost of manufacture. If the hot air is circulated through a multiplicity of small perforations, the air flow encounters a markedly increased resistance and, therefore, the amount of hot air introduced into the heating chamber is reduced, with the result that, notwithstanding a substantial increase of temperature within the compartment housing the heater means, the temperature in the heating chamber does not increase as might be expected, thus providing only a reduced thermal efficiency.
Moreover, in the prior art cooking appliance of hot air circulating type, the portion of the food located within vicinity of the energy feeding port is directly exposed to hot air flow and is heated intensively to brown the food but the portion of the food located away from this vicinity is exposed only to a reduced flow of hot air and tends to be undercooked or only inadequately browned. Therefore, the velocity of hot air had to be increased in order to avoid such an uneven cooking of food. However, the increased air velocity means that the hot air bombards the food with an additional force, which, in turn, means that the surface of food is subject to a more vigorous evaporation of water and becomes crusted, with the flavor of the food being also adversely affected.